1. Field of the Invention
The present invention relates to a technique for measuring loss distribution along a length of optical fiber in an optical transmission system by using an OTDR (optical time domain reflectometer).
2. Description of the Related Art
The OTDR is used for detecting distance distribution of transmission loss in optical fiber or for locating fiber fault or the like. By using the OTDR, a pulse of light is transmitted down the fiber and the backscattered signal is detected. The intensity of the backscattered light provides a measure of the loss in the optical fiber, and the time between transmitting the pulse and obtaining the backscattered light provides a measure of distance in the optical fiber.
There are two types of transmission systems using optical fiber. One type (referred to as “relay transmission system” hereinafter) is a transmission system in which relays are provided at regular intervals on a transmission line between both terminal stations. Another type (referred to as “no-relay transmission system” hereinafter) is a transmission system in which no relay is provided on the transmission line. As to the relay transmission system, an optical transmission system capable of long distance measurement can be realized by amplifying light by using EDF (erbium doped fiber) in each relay. In addition, as to the relay transmission system, long distance measurement across the relay can be performed by using C-OTDR that supports coherent detection.
On the other hand, as to the no-relay system, a low cost system can be realized. However, transmission distance is limited even though Raman amplification is performed since no relay is provided. Therefore, transmission distance of OTDR signal light is limited so that measurement distance from a terminal station that has an OTDR is limited. As a form of the no-relay transmission system for increasing transmission distance, a remote pump system is used in which a remote amplifier including EDF is provided on a transmission line at a position a predetermined distance apart from a terminal station. However, from the viewpoint of measurement by OTDR, since absorption loss by EDF is large at 1550 nm band that is generally used as OTDR signal light, it is difficult to measure loss distribution beyond EDF from a terminal station that performs OTDR measurement. In addition, even when light of 1650 nm band that can pass through the EDF is used, good S/N ratio cannot be obtained since transmission loss by optical fiber is large at the 1650 nm band. Therefore, it is difficult to measure loss distribution along a long distance length of optical fiber by the no-relay transmission system.
As mentioned above, as to the no-relay transmission system, there is a problem in that OTDR measurement of loss distribution cannot be performed for a long span optical transmission line.
Following are example documents relating to the technical field of the present invention:
(1) Huai H. Kee et al. “Extended-range optical time domain-reflectometry system at 1.65 μm based on delayed Raman amplification”, Optical Letters Vol. 23, No. 5 Mar. 1, 1998, pp. 249-351,
(2) E. Cottino et al. “DYNAMIC RANGE INCREASE OF 1625 nm MONITORING SYSTEMS”, International Wire & Cable Symposium Proceedings 1995, pp. 654-661.